Turbine overspeed trip anticipator

ABSTRACT

Turbine control system with conventional overspeed trip for stop valves and governor for control valves includes an additional trip device which temporarily closes stop valves to anticipate overspeed tripping, but reopens them to prevent permanent tripping if the governor functions to bring overspeed under control. 1CROSS REFERENCE

United States Patent 72 Inventor Markus A. Eggenberger 3,060,692 10/190211dw;11'I;I1.I............... 60/73 Schenectady,N.Y. 3,094,041 6/1963Wagner 137/36 X [211 App]. No. 666,526 3,097,488 7/1963 Eggenberger60/73 [22] Filed June 21, 1967 3,097,490 7/1963 Callan 60/73 [45]Patented Oct. 19, 1971 3,198,954 8/1965 Eggenberger 290/40 [73] AssigneeGeneral Electric Company 3,211,957 10/1965 Carson 137/31 XContinuation-impart of application Ser. No. FOREIGN PATENTS 1965 555,5417/1932 Germany..................... 60/105 Primary Examiner-Clarence R.Gordon TURFINE R F TRIP ANTICWATOR Attorneys-William C. Crutcher, FrankL. Neuhauser, Oscar 9 Claims, 7 Drawing Flgs- B. Waddell and Melvin M.Goldenberg 521 u.s.c1 2110 40, 60/73, 60/105, 317/19, 317/31, 415/10,415/32 [51] Int. Cl 1 1 "02p 9/04 [50] Field oi Search 137/21, 22,

24, 27, 26, 1 35, 36160/73, 290/40; ABSTRACT: Turbine control systemwith conventional over- 415/10, l3, l6, 30, 32; 317/19 speed trip forstop valves and governor for control valves includes an additional tripdevice which temporarily closes stop [56] References Cited valves toanticipate overspeed tripping, but reopens them to UNITED STATES PATENTSprevent permanent tripping if the governor functions tobring 3,427,4641/1969 Watson 290/40 overspeed under control.

VALVE I I l" SPEED 1 4 1 /CONTROL 9 11) TEAM 15 I 18' Low GENERATOR |2 SE PBESSUR; GE NERATO R OVET SSEED TURBINE 3 TURBINE TURBINE l I V TRIPCONTROL ANTICIPATOR -f T;

1 INTERCEPT I L VALVE "SE 'T CONDE R L 18b -10 I R H AT REHEATER V 17STOP VALVE l PATENTEDucnsmn 3.614.457

SHEET 10F 4 L f lau g r A SPEED A A 4 4 /CONTROL H STEAM l5 Ha I RMGENERATOR |2 g g g MPBESSQRE GENERATOR OVERSPEED T RBI E WP 9Y CONT Rm uN TURBI E TRIP v ANTICIPATOR V VALVES LOAD I L. I '-r-- R g 5 L 8 b-CONDENSE Io REHEATER n STOP VALVE 29 REHEAT STOP l VALVE TOLV. RELAYCONTROL VALVE RELAY AUTOMATIC R MANUAL RESET STOP VALVE RELAY TURBINEINVENTOR MARKUS A. EGGENBERGER,

BY 116M LIIC ATTnDnL-v /o LOAD PRIOR TO FULL LOSS OF LOAD INVENTORIMARKUS A. EGGENBERGER,

BY c/Ai HIS ATTORNEY.

TURBINE OVERSPEED TRIP ANTICIPA'IOR CROSS REFERENCE CROSS REFERENCE TORELATED APPLICATIONS This application is a continuation-in-part of myabandoned application Ser. No. 468,887 filed July I, l965 and assignedto the present assignee.

BACKGROUND OF THE INVENTION This invention relates to an improvedturbine control system for preventing dangerous overspeed of theturbine, and more particularly, the invention relates to a controlsystem for anticipating and sometimes forestalling operation of theconventional overspeed emergency trip under conditions which wouldresult in dangerous overspeed should the normal speed control systemfail to control the turbine speed.

Improvements in large-capacity turbine-generators have resulted ingreater difficulties in controlling speed upon loss of load, due to thegreater entrained steam energy and higher power density relative to theinertia of the rotating parts. Faster-closing stop valves havecontributed substantially to reducing the emergency speed rise duringclosing of these valves. However, improvements in stop-valve-closingspeeds are being fully exploited and additional anticipatory measuresare required to prevent overspeed.

Another approach has been to anticipate overspeed by closing the valvesunder condition of power-load unbalance. One such arrangement isdisclosed in US. Pat. No. 3,198,954 granted Aug. 3, 1965 to M. A.Eggenberger and P. H. Troutman, and assigned to the assignee of thepresent application.

A conventional steam turbine-generator has two lines of defense againstoverspeed. There is normally a set of control valves" operated by thespeed governor. Ahead of the control valves in series is a stop valveactuated by a simple, very reliable emergency governor, preferablycalled an overspeed trip. In a reheat steam turbine, the same speedgovernor and emergency governor may also actuate respectively a set ofintercept valves" and reheat stop valves these having much the samecontrol functions as the aforesaid control valves and stop valve.

The turbine is usually designed so that the overspeed trip must bemanually reset by the operator and since this is undesirable for manyreasons, including time required to put the turbine-generator back inservice, the rules of design provide that the unit shall be capable ofrejecting maximum guaranteed load without tripping the overspeed trip.Other design rules, which will be familiar to those skilled in the art,provide that the speed setting of the overspeed trip be a specifiedincrement above the expected peak speed on loss of load. The above tworules necessarily mean that the turbine will be at a speed substantiallyabove its normal operating speed at the time the overspeed trip isactuated should a need for it occur. Thereafter, the turbine reacheseven greater overspeed before the closing of the stop and reheat stopvalves reduce the steam flow to zero.

The greater the load which is being carried before load is lost, themore rapidly will the rotor accelerate because of the larger unbalanceof torques on the rotors.

Accordingly, one object of the present invention is to provide a devicewhich will anticipate imminent actuation of the overspeed trip and willact in its stead to prevent the turbine from exceeding a specified peakspeed upon loss of load, should the control valves or intercept valvesfail to control the speed.

Another object of the invention is to provide a device to close the stopvalves at a speed lower than the overspeed trip setting, but tosubsequently reopen the stop valves again should the normalspeed-control system have functioned properly.

Still another object of the invention is to provide an improvedoverspeed trip anticipator which is adaptive in accordance with theconditions of load prevailing on the turbinegenerator just prior to lossof load. Yet another object of the invention is to provide an improveddevice for limiting peak speed to a selected value without causing theoverspeed trip to function while at the same time protecting the turbineagainst failure of the conventional overspeed system.

The subject matter of the invention is particularly pointed out anddistinctly claimed in the concluding portion of the specification.

The invention, however, both as to organization and method of practice,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawing in which:

FIG. 1 is a simplified schematic diagram of a typical reheatturbine-generator powerplant and control system employing the invention,

FIG. 2 is a simplified diagram showing the trip anticipator in itssimplest form added to a conventional control system,

FIG. 2a is a graph illustrating the operation of the device shown inFIG. 2,

FIG. 3 is a modified form of the invention illustrating an adaptive tripanticipator,

FIG. 3a is a graph illustrating the operation of the device shown inFIG. 3, and

FIGS. 4 and 5 illustrate a modified form of the invention for anelectrohydraulic control system.

SUMMARY OF THE INVENTION Briefly stated, the invention is practiced byproviding speed responsive means to actuate the stop valves at a speedbelow the setting of the conventional overspeed trip, in such a mannerthat the stop valves may reopen should the normal speed-control system,using control valves and intercept valves, function properly. A modifiedform of the invention adapts the trip anticipator speed setting inaccordance with the load on the turbine to positively limit peak speedbelow that which would occur upon actuation of the overspeed trip.

DESCRIPTION OF MECHANICAL-HYDRAULIC TRIP ANTICIPATOR Referring now toFIG. I of the drawing, the steam flows from steam generator 1 throughmain stop valve 2, a set of control valves 3, high-pressure turbines 4,and is reheated in reheater 5. The steam then flows throughreheat stopvalve 6, intercept valve 7, intennediate-pressure turbine 8,low-pressure turbine 9 to condenser 10. Turbines 4, 8, 9 drive a loadsuch as generator II.

The normal speed-control system comprises a speedresponsive device 12,which can be either mechanical or electrical, connected to actuateconventional mechanical-hydraulic or electrohydraulic relays l3, l4actuating control valves 3 and intercept valve 7 respectively.

An emergency governor or overspeed trip 15 is a simple speed-responsivedevice, such as an oflcenter bolt" or similar known device which tripsmain stop valve 2 and reheat stop valve 6 through relays 16, 17respectively. This may be done by dumping oil pressure from a line bymeans of a pilot valve and is well known in the art.

The trip anticipator 18 which is the subject of the present invention isalso connected to actuate relays l6, 17 in a manner to cause the stopvalve 2 and reheat stop valve 6 to close. This is accomplished through aseparate automatically resetting type of control to the stop valve relayl6, 17 as indicated by the dotted lines 18a, 18b. Trip anticipator 18may employ a third speed-responsive device or it may be convenientlyactuated by the main governor 12 as indicated by dotted line 19.

In order to simplify the description of the trip anticipator and itsrelation to the conventional turbine-generator control system, FIGS. 2through 5 omit the reheat portion of the steam cycle and the portions ofthe control system which pertain to the reheat stop valve and interceptvalve controlling the reheat steam. The trip anticipator is notnecessarily restricted to use in a reheat turbine, but it is shown inthat environment in FIG. 1 because it is most useful in large-capacityturbinegenerator units which would normally employ reheat. However, anunderstanding of its operation is facilitated by examining the tripanticipator in its simplest form in FIG. 2, where the control elementshave the same reference numbers as in FIG. 1, insofar as possible,although the figure is arranged in a slightly different manner.

Normal speed/load control is obtained by means of the speed governor 12operating the control valves 3 through control valve relay 13. This isachieved through a speed-goveming servomechanism shown generally as 20comprising a pilot valve 2i, a single acting servopiston 27 withcylinder 22, and a restoring lever 23 attached to an adjustable fulcrum24. The stem 25 of pilot valve 21 is positioned up or down by themovements of speed governor 12 through a self-aligning link 26 tocontrol the admission of hydraulic fluid from a pressure source toservomotor 22. The resulting movement of the servopiston 27 causes lever23 to position slidable sleeve 28 of the pilot valve. Thus the movementsof servopiston 27 correspond with the position of the speed governor,but the force and the stroke is greatly multiplied. Adjustment inaccordance with desired load is accomplished by moving the adjustablefulcrum 24. The movements of servopiston 27 may also be communicated tothe intercept valve relay (see FIG. 1) as indicated by the dot-dash line29.

A simplified form of emergency overspeed trip is illustrated in FIG. 2by means of a speed responsive offcenter bolt device connected to rotatewith the high-pressure turbine shaft. Device 15 is arranged to actuate apivotable trip finger 30 so as to release stem 31 of a spring-loadeddump valve 32 in such a manner as to release oil pressure in conduit 33.Means (not shown) are provided to manually reset dump valve 31 after ithas tripped, as will be understood by those skilled in the art.

Stop valve relay 16 which is a hydraulic servomechanism arranged tocontrol stop valve 2 in an offer! manner, is provided with two incominghydraulic lines shown as a manual reset" line 34 and automatic resetline 35. Hydraulic pressure is required in both lines to hold stop valve2 in an open position. Dumping of oil from line 34 necessitates manualresetting of stop valve relay to. However, dumping of oil pressure fromline 35 and subsequent restoration of hydraulic pressure will cause thestop valve 2 to first close and then reopen.

It will be understood that the foregoing description of the conventionalspeed-governing system through the control valves and the emergencyoverspeed trip system with the stop valve is described in very simpleschematic form and that many other devices such as load limit,speed-regulation adjustment, and various other adjusting features wouldbe necessary in an actual steam turbine control system.

The trip anticipator, shown generally as 58, consists of a tripanticipator pilot valve 36 and a trip-anticipator relay 37. Pilot valve36 has a valve stem 38 which is positioned in accordance with turbinespeed. This is conveniently accomplished by using the speed signal fromthe main governor l2 and arranging the pilot valve stem 38 so that it isan extension of pilot valve stem 25. However, it will be obvious that asupplementary speed-responsive means in addition to main governor 12might be employed to position pilot valve stem 38.

Pilot valve sleeve 39 is adjustable and may be raised or lowered todetermine the point at which hydraulic fluid in conduit 60 will bedumped to actuate the trip-anticipator relay 37, as should be apparentfrom the drawing. Downward movement of pilot valve stem 38 upon increasein speed causes fluid to be discharged from conduit 40 at the speedwhich is set using the adjustable sleeve 39.

Trip-anticipator relay 37 is in reality a pressure-actuated three-wayvalve and comprises a sleeve 41 and a spring-loaded stem 42. An upperland 43 acts as a piston to hold stem 42 downward against the springpressure in the position shown so that ports M and 45 communicate whenthe turbine is at a speed below that at which the trip anticipator isactuated. On loss of pressure in conduit 40, stem 42 will rise anddischarge fluid from the automatic reset line 35 through port 45 of therelay 37. On the other hand, fluid will not be discharged through port44 of the relay 37, but hydraulic pressure will be maintained in themanual reset line 34 which is supplied by line 33 via the emergency dumpvalve 32.

Operation of FIG. 2 will be described by reference to the graph of FIG.2a, on which the percent of rated load prior to full loss of load isplotted as the abscissa and on which the percent of rated speed isplotted as the ordinate. The dashed line 46 represents normal overspeed"which is the peak speed which will result if load as indicated on theabscissa is lost and the main governor l2 acts in the expected mannerthrough servomechanism 20 to close the control valves 3 so as to limitthe overspeed. It will be observed that if I00 percent load is beingcarried and is suddenly lost, the normal overspeed curve 46 indicatesthat approximately llO percent of rated speed will occur before theturbine speed levels off and starts decreasing.

Curve 47 illustrates the peak speed which will occur if the conventionaloverspeed trip is actuated at a speed setting indicated by dot-dash line48 (assuming that the normal speed control through the control valvesfailed to function). Curve 48 is cross-hatched to indicate the margin oftripping which normally occurs with this type of governor, and it isalso curved to illustrate the delay in tripping which takes place due toacceleration effects on the trip device at large load losses. Customaryrequirements applicable to large turbine-generators require that speedsetting 48 of the overspeed trip be higher than the normal overspeedcurve 46 by a specified in- .crement so that loss of load will notresult in actuating the overspeed trip. It will be observed that if fullload is lost and if the control valves fail for some reason to controlturbine overspeed in accordance with curve 46, then the overspeed tripwill be actuated and the peak speed, as indicated by curve 47, willresult in excessive speeds of the turbine.

Superimposed on the graph of FIG. 2a is a third set of curves inaccordance with the present invention, designated as thetrip-anticipator setting 49, shown in dot-dash lines, and thetrip-anticipator peak-speed curve 50 shown in dashed lines. Thetrip-anticipator setting 49 is adjusted by means of the pilot valvesleeve 39 in such a manner that the peak-speed curve 56) resultingtherefrom will terminate at a desired peak speed upon loss of full load,in this case I20 percent of rated speed. It will be observed that thetrip-anticipator setting curve 39 lies below curve 48 and tripanticipation will occur before the conventional overspeed trip mechanismis put into play.

Referring back to FIG. 2, assume that full load is being carried andthat full load is suddenly lost and that, for some reason, the controlvalves fail to control turbine overspeed in the expected manner shown bycurve 46 on FIG. 2a. Lowering of pilot valve stem 38 will causehydraulic fluid to be discharged from conduit 40, causing pilot valvestem 42 of the trip-anticipator relay 37 to rise. Fluid is dumped fromthe automatic reset line 35 which causes stop valve relay 16 to closestop valve 2. Similarly, fluid will also be dumped from the automaticreset line 35a leading to the reheat stop valve, causing it to closealso.

Assume now that speed is brought under control before the turbinereaches the overspeed trip setting (curve 48 of FIG. 2a), decrease inspeed will cause anticipator pilot valve stem 38 to rise and reestablishfluid pressure in conduit 40, causing the trip-anticipator relay stem 42to resume its position shown in FIG. 2. This action reestablishes fluidpressure in line 35 and causes the stop valve relay 16 to reopen stopvalves 2. The foregoing action has taken place without dumping of oilpressure from the manual reset lines 34 or tripping of the emergencygovernor pilot valve 32. The control over the turbine is reestablishedin terms of the control valves 3, therefore, without actuating theoverspeed trip.

If, on the other hand, the normal speed-goveming system does notfunction, and the speed continues to rise. the overspeed trip will beactuated in the usual manner, causing the stop valve and reheat stopvalve to remain closed. In this case, fluid will also be dumped from themanual reset lines 34, 340 by means of the overspeed trip pilot valve32. However, it should be particularly noted that since the closing ofthe stop valves was anticipated and carried out at an earlier period intime, the peak turbine speed will not go any higher than curve 50.Without a trip-ancitipator device, peak speed would reach the valuesindicated by curve 47, which would result in an excessive overspeed forthe turbine.

In the arrangement previously described in connection with FIGS. 2 and2a, it was observed that the trip-anticipator setting 49 was fixed asdetermined by the setting of pilot valve sleeve 39 on thetrip-anticipator pilot valve. FIG. 3 and the accompanying graph, FIG.3a, illustrate a modification, wherein the trip-anticipator speedsetting is caused to be adaptive to reflect the load being carried bythe turbine. Here the setting of the trip-anticipator pilot valve sleeveis caused to reduce the trip-anticipator speed setting at high values ofturbine load. In this manner, anticipated closing of the stop valveswill take place even earlier and in some cases even before the turbinereaches its normal overspeed corresponding to this load level.

In FIG. 3, the same elements are indicated by the same referencenumerals as in FIG. 2, with the following exceptions.

Pilot valve sleeve 39 of the trip-anticipator pilot valve 36 ispositioned by means of a lever 51 fulcrumed at 52 when it moves upwardsufficiently to hit an abutment 53 on the pilot valve sleeve stem. Whenlever 51 is below abutment 53, the sleeve 39 is held in a fixed positionby spring 54 in accordance with aspeed adjustment depicted by nut 55.

Theright-hand end of lever 51 is raised by means of an extension 56 onthe servomotor piston 27 at a point determined by the adjusting screw57.

A dash pot 58 serves as a memory device to hold the right-hand end oflever 51 in an elevated position for a few seconds in the event thatextension 56 is suddenly withdrawn to a downward position, as would bethe case in a sudden increase of speed upon loss of load.

As will be known to those skilled in the art, the position of servomotorpiston 27 when the turbine-generator is synchronized and carrying loadwith other turbine-generators will be proportional to the load beingcarried at rated speed. On the other hand, when load is lost, theservomotor piston 27 will descend rapidly because of the increase inspeed.

Reference to FIG. 3a of the graph will illustrate the operation of themodification shown in FIG. 3 with the adaptive trip-anticipator setting.The reference numbers correspond to those on the graph of FIG. 2a withthe following exceptions.

Curve 5) illustrates the speed setting at which the trip-anticipatorpilot valve is actuated to dump hydraulic fluid from line 40. Thecorresponding peak speed which would be reached by the turbine isillustrated by curve 60. It will be noticed that at point 61, thetrip-anticipator speed setting is gradually lowered as the loadincreases, at a rate which results in a flat peak-speed curve portion62. This results in limiting turbine overspeed to a selected valve.Point 61 on the graph is determined by the setting of adjustment screw57 which, in turn, determines the point at which lever Sll commences tobe raised. The slope of curve 59 beyond point 61 is, of course,determined by the mechanical advantage of lever 31, and is predeterminedso that a flat peak-speed curve at 62 results. The elements in FIG. 3are shown as positioned at a load of approximately 85 percent whenadjusting screw 57 just touches lever 53. Subsequent increase in loadwill raise the pilot valve sleeve 39 to lower the setting at whichsubsequent increase in speed will cause the trip anticipator tofunction.

The operation of the modification shown in FIGS. 3 and 3a is the same aspreviously described with respect to action of the trip anticipator atlow values of load. Above the setting of 85 percent load in the exampleshown, the turbine steam-control valves will be opened to such an extentthat lever 51 will be raised by the extension 56 on the servomotorpiston to lower the trip-anticipator speed setting. Upon loss of load,the extension 56 will be withdrawn, but dashpot 58 will hold lever 51 inan elevated position long enough for trip-anticipator pilot valve 36 todump fluid from conduit 40. This will evacuate the automatic reset lines35, 35a as before and cause the stop valves to close. Restoration offluid pressure in lines 35, 35a will reopen the stop valves should thecontrol valves and intercept valves function in the expected manner. Inany event, however, the turbine peak speed will not exceed thatindicated as on curve portion 62 in FIG. 3a, because tripping of theconventional overspeed trip has been anticipated, and the stop valveshave already been closed when overspeed tripping occurs. At very highvalues of load where the trip-anticipator setting crosses the normaloverspeed curve, the stop valves will always close, but they will reopenif the normal speedgoverning system functions properly without actuatingthe overspeed trip.

DESCRIPTION OF ELECTROHYDRAULIC TRIP ANTICIPATOR As indicatedpreviously, the trip anticipator is adaptable for use either with amechanical or an electrical turbine control system and can receive itsspeed signal from an independent speed-responsive device. FIGS. 4 and 5illustrate a modification of the invention, in simplified form, for useon an electrohydraulic control system of the type more fully describedin Us. Pat. No. 3,097,488 issued to M. A. Eggenberger et al. on July 16,1963. The portions of the control for the reheat portion of the steamcycle are omitted for simplicity.

FIG. 4, in so far as it differs from the preceding figures, illustratesan electrohydraulic control for the steam control valves 3 comprising aspeed control 70 and a load control 75. The speed signal is obtainedfrom a toothed wheel 72 on the turbine shaft which initiates pulses in aspeed sensor 73 as before. The actual speed is compared with a desiredor reference speed set with knob 74 and the resulting error signal,after suitable modification, is supplied to the load control unit 75.There a desired load demand or load reference signal is superimposed onthe speed signal so as to control the valves through relays vI3, inaccordance with a conventional control system.

According to the present invention, an additional speed sensor '77supplies a speed signal to the trip anticipator 78. The trip-anticipatorspeed setting is adjusted as desired with knob 79, this being analogousto the setting obtained with the pilot valve sleeve 39 in FIG. 2.Actuation of the trip anticipator dumps fluid from the automatic resetline leading to the stop valve relay to as before. This is a differenttype of tripping action than is obtained by the overspeed trip 15 whendumping fluid from the manual reset line 81. Reestablishment of pressurein line 86} will cause the stop valve 2 to automatically reopen, whereasmanual resetting is necessary when fluid is dumped from line 81.

Reference to FIG. 5 of the drawing shows a simplified form of electricaltrip-anticipator actuator. A trigger amplifier 82 is arranged to actuatea solenoid dump valve 83 to dump fluid from the automatic reset line 80when the summed voltage at the input to the trigger amplifier becomesnegative. Dump valve 83 is analogous to valve 37 in FIG. 2.

An adjustable positive bias voltage is applied to line 84 by means ofthe trip-anticipator speed setting knob 79. A counterbalancing,increasingly negative voltage is applied to line 35 as the turbine speedincreases. At rated speed, the positive voltage on line 841 exceeds thenegative voltage on line 85 by a selected amount, in accordance with thesetting of knob 79. To this extent, the trigger amplifier will beactuated if the turbine overspeeds to such an extent that the negativevoltage exceeds the positive bias and trips the dump valve 83. Theaction is thus the same as that of the mechanical-hydraulic system shownin FIG. 2 and its operation is as shown in FIG. 2a.

In order to provide the additional load adaptive feature described inthe mechanical-hydraulic version of FIG. 3 and the operation of FIG. 3a,an additional input voltage to the trigger amplifier 82 is providedwhich, above a certain load, applies an increasingly negative voltage online 86 as the load increases. This reduces the trip-anticipator settingabove a selected load as indicated in FIG. 3".

Although the FIG. 3 mechanical-hydraulic arrangement reduced thetrip-anticipator setting in accordance with actual load on the turbine(as imposed by the position of stop 57 in FIG. 3), the electrohydraulicversion reduces the trip-anticipator setting in accordance with the loaddemand or load reference signal setting on the load control unit 75. Asthe load reference is increased by turning knob 76, an increasinglynegative voltage is applied on line 87. This voltage is blocked by Zenerdiode 88, until the load setting exceeds a predetermined value, hereabout 70 percent load, after which the voltage on lead 86 becomesincreasing negative and serves to lower the degree of overspeed at whichthe trip anticipator is actuated.

One of the accessory devices on the electrohydraulic control may includemeans to suddenly reduce the load reference signal, such as a relay 89.To prevent the negative signal on line 86 from dropping suddenly tozero, an RC circuit 90 delays the sudden loss of the signal, acting in asimilar manner to the dash pot 58 of FIG. 3.

Thus it will be seen that means have been provided which anticipateoverspeed tripping by closing the stop valves in a time sufficient toprevent a peak speed exceeding a desired amount. If the normalspeed-goveming system functions as it should, then the stop valves willreopen. If the normal speedgoverning system does not function, then thestop valves will already be closed when the conventional overspeed tripmechanism is actuated.

The additional modification of an adaptive trip-anticipator setting tolower the setting at high values of actual load or load demand is usefulfor very large capacity turbine-generator units in which the high-powerdensity could result in excessive overspeeds if the conventionalemergency overspeed trip were the only device used to control overspeedin the event of failure of the normal speed-goveming system.

What is claimed as new and desired to secure by Letters Patent of theUnited States is:

1. in a turbine of the type having first and second valve meansconnected in series controlling the flow of motive fluid thereto andhaving normal speed-governing means actuating said first valve means andalso having emergency speedgoverning means arranged to close the secondvalve means at an overspeed trip setting, the improvement comprising:

overspeed trip-anticipator means arranged to close said second valvemeans at a selected trip-anticipator speed setting below the overspeedtrip setting of the emergency speed-governing means and to reopen saidsecond valve means if the turbine speed thereafter falls below saidtripanticipator speed setting without having first exceeded theoverspeed trip setting of the emergency speed governing means, and

means arranged to reduce the trip-anticipator speed setting above aselected turbine load.

2. The combination according to claim 1 wherein said means arranged toreduce the trip-anticipator speed setting are load responsive.

3. The combination according to claim 1 wherein said means arranged toreduce the trip-anticipator speed setting are responsive to turbine loadsetting.

4. In a turbine of the type having first and second valve meansconnected in series to control the flow of motive fluid thereto, thecombination of:

first governing means having first speed-responsive means and normallycontrolling said first valve means,

second governing means having second speed-responsive means andcontrolling said second valve means so as to close the second valves atan overspeed trip setting,

third governing means connected to temporarily close said second valvemeans in response to a trip-anticipator speed setting lower than theoverspeed tri speed setting whereby closing of the second valves y thesecon governing means is anticipated by the third governing means shouldthe first governing means fail to control speed.

means arranged to reduce the trip-anticipator speed setting above aselected turbine load, and

means for temporarily retaining said trip-anticipator speed setting uponsudden loss of load.

5. The combination according to claim 4 wherein said second governingmeans dumps hydraulic fluid to cause the second valves to close andwherein said third governing means comprises relay means arranged todump hydraulic fluid from a second line upon increase in speed and toreestablish hydraulic pressure in the second line upon decrease inspeed, said second line being also connected to actuate the secondvalves.

6. The combination according to claim 4 wherein said third governingmeans employs a third independent speed-responsive means.

7. In a reheat steam turbine-generator powerplant having a high-pressureturbine and an intermediate-pressure turbine employing reheated steam,the combination of:

main stop valve and main control valves connected in series to controlthe flow of high-pressure steam to the highpressure turbine,

reheat stop valve and intercept valves connected in series to controlthe flow of reheated steam to the intermediate pressure turbine,

main governing means connected to control admission of steam throughsaid control valves and intercept valves in response to turbine speed,emergency governor means connected to close said main stop valve andsaid reheat stop valve at an overspeed trip setting,

trip-anticipator means connected to temporarily close the main stopvalve and the reheat stop valve upon reaching a selectedtrip-anticipator speed setting lower than the overspeed trip speedsetting and to reopen said stop valves if the overspeed trip setting isnot attained.

means arranged to reduce the trip-anticipator speed setting above aselected turbine load, and

means to retain the trip-anticipator speed setting temporarily uponsudden loss of load.

8. The combination according to claim 7 wherein said trip anticipator isactuated in response to a speed indication from said main governingmeans.

9. The combination according to claim 7 wherein said trip anticipator isactuated in response to a speed indication from an independentelectrical speed responsive means.

1. In a turbine of the type having first and second valve meansconnected in series controlling the flow of motive fluid thereto andhaving normal speed-governing means actuating said first valve means andalso having emergency speed-governing means arranged to close the secondvalve means at an overspeed trip setting, the improvement comprising:overspeed trip-anticipator means arranged to close said second valvemeans at a selected trip-anticipator speed setting below the overspeedtrip setting of the emergency speed-governing means and to reopen saidsecond valve means if the turbine speed thereafter falls below saidtrip-anticipator speed setting without having first exceeded theoverspeed trip setting of the emergency speed governing means, and meansarranged to reduce the trip-anticipator speed setting above a selectedturbine load.
 2. The combination according to claim 1 wherein said meansarranged to reduce the trip-anticipator speed setting are loadresponsive.
 3. The combination according to claim 1 wherein said meansarranged to reduce the trip-anticipator speed setting are responsive toturbine load setting.
 4. In a turbine of the type having first andsecond valve means connected in series to control the flow of motivefluid thereto, the combination of: first governing means having firstspeed-responsive means and normally controlling said first valve means,second governing means having second speed-responsive means andcontrolling said second valve means so as to close the second valves atan overspeed trip setting, third governing means connected totemporarily close said second valve means in response to atrip-anticipator speed setting lower than the overspeed trip speedsetting, whereby closing of the second valves by the second governingmeans is anticipated by the third governing means should the firstgoverning means fail to control speed. means arranged to reduce thetrip-anticipator speed setting above a selected turbine load, and meansfor temporarily retaining said trip-anticipator speed setting uponsudden loss of load.
 5. The combination according to claim 4 whereinsaid second governing means dumps hydraulic fluid to cause the secondvalves to close and wherein said third governing means comprises relaymeans arranged to dump hydraulic fluid from a second line upon increasein speed and to reestablish hydraulic pressure in the second line upondecrease in speed, said second line being also connected to actuate thesecond valves.
 6. The combination according to claim 4 wherein saidthird governing means employs a third independent speed-responsivemeans.
 7. In a reheat steam turbine-generator powerplant having ahigh-pressure turbine and an intermediate-pressure turbine employingreheated steam, the combination of: main stop valve and main controlvalves connected in series to control the flow of high-pressure steam tothe high-pressure turbine, reheat stop valve and intercept valvesconnected in series to control the flow of reheated steam to theintermediate pressure turbine, main governing means connected to controladmission of steam through said control valves and intercept valves inresponse to turbine speed, emergency governor means connected to closesaid main stop valve and said reheat stop valve at an overspeed tripsetting, trip-anticipator means connected to temporarily close the mainstop valve and the reheAt stop valve upon reaching a selectedtrip-anticipator speed setting lower than the overspeed trip speedsetting and to reopen said stop valves if the overspeed trip setting isnot attained. means arranged to reduce the trip-anticipator speedsetting above a selected turbine load, and means to retain thetrip-anticipator speed setting temporarily upon sudden loss of load. 8.The combination according to claim 7 wherein said trip anticipator isactuated in response to a speed indication from said main governingmeans.
 9. The combination according to claim 7 wherein said tripanticipator is actuated in response to a speed indication from anindependent electrical speed responsive means.